High speed data processing machines typically generate control words which include an operation code OPCODE and an error code for identifying errors which occur in the OPCODE. The error code is typically a parity bit. The control words are supplied to numerous locations within the data processing machine for execution of various subparts of the desired operation. Errors can occur in any copy of the control word at any of the numerous locations.
In high speed machines, errors in control words are typically detected too late to prevent action by the machine on the erroneous control word. Therefore, it is necessary to detect the occurrence of an error as soon as possible and to classify the error according to the possible damage caused to the machine. Some control word errors are catastrophic, in that the associated control functions must not be performed unintentionally or must complete successfully in order to assure integrity of the machine.
Therefore, there is a need to detect the occurrence of errors in the multiple copies of the control word and to identify those errors that may fall within the catastrophic class of errors. Upon detection of errors that fall within this catastrophic class, the machine can react to mitigate the effect. For errors that fall outside the catastrophic class, the machine can typically continue to process following control words.
In this situation involving multiple copies of control words, prior machines might misclassify errors; such that any error in a copy of a control word had to be considered catastrophic leading to unneeded machine interruptions.